Single in-line memory modules (SIMM) represent a high density, low profile single in-line package for electronic components such as dynamic random access memory integrated circuit components. A plurality of these components can be mounted in line on a circuit panel whose height is little more than the length of the components themselves. The circuit panels can in turn be mounted on a printed circuit board daughtercard which can then be mounted on a printed circuit board mothercard. The spacing between adjacent daughtercards would then need to be only slightly greater than the height of the individual circuit panels or single in-line memory modules.
One approach for mounting single in-line memory modules on a daughterboard would be to employ plug in leads adjacent to one edge of the circuit panel. These plug in leads can then be connected to conventional printed circuit board contacts such as miniature spring contacts.
Sockets or connectors containing a plurality of contacts can also be used to interconnect single in-line memory modules on a printed circuit board. For example, U.S. Pat. No. 4,737,120 discloses an electrical connector of the type suitable for use with a single in-line memory module in which a zero or low insertion force interconnection is established between the terminals and the pads on the circuit panel. The circuit panel is inserted at a angle and then cammed into position. The insulative housing on the connector provides a stop to hold the circuit panel in position. Other low insertion force connectors are disclosed in U.S. Pat. Nos. 4,136,917; 4,575,172; 4,826,446 and 4,832,617. Another socket of this type is shown in U.S. patent application Ser. No. 07/398,795 filed Aug. 24, 1989. The contact terminals in each of these patents is edge stamped. Sockets using terminals of this type are suitable for use on center line spacings on the order of 0.050 inches.
For conventional zero or low insertion force single in-line memory module sockets, integrally molded plastic latches are normally used to hold the modules in position. The configuration of the latch members provides the latch members with the resilient characteristics required in order to allow the latch members to cooperate with the daughter board. The latch members cooperate with securing members to maintain the daughter board in electrical engagement with the terminals of the connector. The securing members are generally molded posts which are separate from the latch members, and have molded tabs extending therefrom. The tabs cooperate with openings in the daughter card to maintain the daughter card in position.
However, several problems are associated with the configuration of the latch member described above. The most common failure mode for plastic latches is caused by the lack of wear resistance on the camming surfaces of the plastic latch hooks. These hooks can also be sheared, partially or completely, if the edges of the module P.C. board are sharp. Shearing would also occur if the module P.C. board is excessively long and drives the latch against the latch stop. This latch sop on conventional plastic housings is to prevent the latch from being overstressed, however, if deflection is retarded at a certain point and the hook is placed in shear.
The plastic latches can also be broken if the outward load is excessive, such as impact against the module, or if the operator pulls outward before deflecting the latches enough to disengage the hook from the edges of the modules. Since these connectors are designed for approximately twenty-five insertions and withdrawals, the likelihood of excessive loads being placed on the plastic latches is significant. Stress relaxation is also more of a problem with plastics, suitable for use with single in line modules, than for more resilient materials. Slight permanent set also occurs during the first cycle to full deflection of the plastic latch. Slight set during the additional (24) cycles can also occur. Consequently, as the memory module circuit panels can vary in size, and still fall within the tolerance limits for the connector, it is possible that a relatively large board will be inserted into the slots, and then be followed by a relatively small board. The insertion of the large board into the slot can cause the plastic latch to take a permanent set, so that as the small board is inserted, the latch will not be effective in maintaining the board in the slot, resulting in an ineffective connector.
Another problem with insulative housings having integrally molded latch members and securing members is that not all insulative materials, otherwise suitable for socket housings, can be used to mold housings having deflectable latch arms and rigid securing tabs. Typically, the plastics suitable for use in a connector housing with deflectable integrally molded latch arms, are more expensive than other materials. Plastics that would provide molded latches that would exhibit toughness and resiliency, and little permanent set at room temperatures can lose those performance requirements when subjected to elevated temperatures. It is essential the connector body of the single in line memory module connectors or sockets remain stable, without distorting under load. There are liquid crystal polymers which do meet the performance criterion for single in line memory module connector housings. Quite often, additional care must be taken in molding such materials, resulting in additional expense as part of the mold tooling or the cycle of the molding operation. For example, U.S. patent application No. 07/234,362, filed Aug. 18, 1988, discloses steps necessary to mold integral members extending at right angles to the direction of flow of a liquid crystal polymer used in a single in-line memory module socket of this type. Elimination of these orthogonally projecting members, such as integrally molded plastic latches, would simplify the molding of the insulative housings and might even result in the use of less expensive plastics which do not exhibit the resilience otherwise required.
A problem also exists with housings which have securing tabs formed of plastic. The securing tabs cooperate with openings of the daughter board to positively insert the daughter board in the housing and maintain the board therein. Over many cycles, the plastic securing tabs will wear, thereby causing the securing tabs to be ineffective.
One option which avoids the need to use integrally molded plastic latches, is the use of separate metal latch formed of the spring material. A greater deflection is obtained with less set with a metal latch. A metal latch is less likely to shear and wear will be minimal. U.S. patent application No. 07/313,261 filed Feb. 21, 1989. The compliance of that latch is, however, restricted by the fact that is partially anchored at its base. Another problem is that the forces placed upon a metal latch of this type during insertion of the single in-line memory module into the socket and as a result of the movement placed upon the electronic module by the terminal spring contacts, must be transmitted to a relatively fragile housing. The fragility of the housing is in part due to the dimensional constraints placed upon the socket, which results in the necessity to use relatively thin sections in the insulative housing.
A metal latch member, having sufficient compliance for use in a single in-line memory module and permitting simplification of the configuration of the molded insulative housing is therefore quite desirable. The instant invention provides just such a resilient metal latch for use in a single in-line memory module socket.